1. Field of the Invention
The present invention relates to a MOS field effect transistor used as a process control monitor in a semiconductor integrated circuit.
2. Description of the Related Art
FIG. 2A is a schematic sectional view of a process control monitor composed of a conventional MOS transistor. First, a description will be made of the process control monitor with an enhancement type N-channel MOS transistor (hereinafter, abbreviated as an E-type NMOS) as an example. In the E-type NMOS, provided on a P-type semiconductor substrate 1 are an N-type source region 4, an N-type drain region 5, a gate insulating film 2, and a gate electrode 3. Also, provided are a source electrode terminal 4a, a drain electrode terminal 5a, and a gate electrode terminal 3a, each of which is intended to give a potential and is composed of a metal, and a substrate contact terminal 6a to fix a substrate potential.
Further, in a structure generally used for a process control monitor, one of the wirings of the gate electrode 3 is grounded to the P-type semiconductor substrate 1 through an N-type high-concentration region 7 (for example, refer to Introduction to VLSI Design, written by Matsuyama, Y. and Tomisawa, T., published by Kyoritsu Syuppan Co., Ltd. (p. 48)). The N-type high-concentration region 7 and the P-type semiconductor substrate 1 constitute a bulk diode which operates as a protective diode for the gate electrode. This provides an effect that the electrical charges which are charged in the gate electrode 3 in a manufacturing process are discharged to the P-type semiconductor substrate 1 through the bulk diode, which prevents the breakdown or deterioration of the gate insulating film 2 due to the charged electrical charges. This is effective against charging in the manufacturing process after the formation of metal electrode. The reason that the gate electrode 3 is grounded through a bulk diode in a reverse direction is that a positive voltage is applied to the gate electrode terminal 3a at the time of measuring the E-type NMOS. In order to prevent a gate current from flowing through the grounded part at the time of the voltage application, a diode is connected in a reverse direction. Such a protective diode for protecting the gate electrode can also be implemented in a PMOS besides an NMOS by inverting conductivity type.
FIG. 2B is a schematic sectional view of a depletion-type N-channel MOS transistor (hereinafter, abbreviated as a D-type NMOS). In this case, on the other hand, it is preferable not to dispose the above-described gate protecting diode. Since the threshold voltage of the D-type NMOS is negative, a forward bias condition is established at the gate protecting diode, which allows current to flow because the forward voltage, at which current starts flowing in a conventional bulk diode, is quite low, when a negative voltage is applied to the gate electrode terminal 3a. 
However, when a protective diode is not formed for preventing the forward current flow, it is impossible to expel the charge in the gate electrode 3 charged in the manufacturing process. This often causes a breakdown of the gate electrode 3 and a shift in the threshold voltage due to charge damage. On the other hand, since the gate electrode almost always has a structure to contact with a diffusion region in a semiconductor integrated circuit including the D-type NMOS, such deterioration due to charge does not occur. In other words, when a bulk diode is attached to the gate electrode of a D-type NMOS transistor to avoid breakdown and deterioration, the transistor would not play a role as a process monitor for checking the condition of the element in the semiconductor integrated circuit.